Recently, cellular telecommunication networks operating, for example, according to the GSM standard have widely spread.
A) Cellular Network Structure
Generally, such networks consist of a plurality of cells. Within each cell, at least one locally fixed transceiver device is provided. Such a fixed and immobile transceiver device (a first type) named base station BS or base transceiver station BTS, respectively, establishes communication on a radio link (Um interface) to a plurality of mobile transceiver devices (a second type) named mobile station MS present within the cell. The cell architecture is hierarchically organized in that each respective base station BS is linked to a so-called base station controller device BSC, which controls the base station devices BS in the plurality of respective cells. Each such base station controller device BSC, in turn, communicates with a so called mobile switching center MSC, which controls a plurality of the BSC devices and provides for a possibility to access a public switched telephone network PSTN like for example an ISDN network.
The cellular architecture of such telecommunication networks is such that each cell is surrounded by, in general, six neighboring cells each of which is provided with a respective base station device BS. Each base station BS is designed and dimensioned such that its transmission power covers at least the area of the respective cell for establishing radio communication between the base station device BS and mobile stations MS present within the cell. Practically, however, the cell borders can not be assumed to be strict, but the cells have rather to be regarded as partially overlapping each other in terms of the transmission range of the base stations. Consequently, interference problems are likely to occur at cell borders, when a mobile station MS (second type transceiver device) receives signals from more than one base station BS (first type transceiver device) of adjacent cells, as will be detailed further below.
B) Transmission Principle
In such cellular telecommunication networks, there exists a first transmission direction from the first type of transceiver devices, namely the base stations BS, to the mobile stations MS as the second type of transceiver stations. This transmission called downlink DL means that the base station BS sends data and the mobile station MS receives data. A second transmission direction is the so called uplink transmission UL. Then, the mobile station MS sends data to the base station BS and the base station receives the data.
Cellular telecommunication systems operate according to the principle of TDMA, i.e. Time Divisional Multiple Access. This means, that the transmission of data is effected on a respective channel (associated to a predetermined frequency), and the transmission time available on each channel is shared by a number of mobile stations MS. The data as such are transmitted in downlink DL as well as in uplink UL transmission in units of bursts, each burst occurring during a predetermined period of time, a so-called time slot TS. A predetermined number n of time slots TS[j], 0≦j≦n−1 is defined to form a frame F. According to GSM specification, n=8 time slots TS, namely TS[0], . . . , TS[7] form a frame. Thus, at a maximum, n mobile stations may communicate with a base station and vice versa every n-th time slot, assuming that a full rate speech traffic channel is used. However, it has to be noted that in case a half rate speech traffic channel is used, 2*n mobile stations MS may communicate per frame with a base station BS.
Hitherto, a transmission between a base station BS and respective mobile stations MS was arranged such that downlink transmission DL occurred during a first frame F[i], while the reception of the “answers” of the respective mobile stations, i.e. uplink transmission UL, occurred in a subsequent frame F[i+1]. The succession of frames occurs in the so called multi-frame structure, adopting a 26-frame multi-frame for speech and data transmission and a 51-frame multi-frame for transmission of signaling information.
Most recent developments, however, have lead to a transmission principle of TDD/TDMA in cellular networks. That is, within a cellular network operating according to TDMA, time divisional duplex TDD is additionally introduced. In general, time divisional duplex means that sending and receiving of data is performed on a single frequency or channel and is shifted in time. Stated in other words, in connection with the transmission in frame units according to TDMA, within a frame F transmitted on a respective channel, there occurs sending of data in one time slot TS[j] and reception of data in a subsequent time slot TS[j′] within the same frame. In particular, according to the principle of TDD, sending and reception is performed alternately within time slots of a frame. For example, considering a base station transceiver device BS, sending or downlink DL, respectively, is performed in time slots having an odd number, while reception or uplink UL, respectively, occurs in time slots having an even number. FIG. 1 illustrates an example for such an assignment of time slots TS[j] within a frame F in conventional TDD/TDMA transmission. It should however be noted that it is also possible that uplink UL occurs in time slots having an odd number, while downlink DL occurs in time slots having an even number. Alternatively, in TDD operation it is also conceivable to adopt a different alternating time division between uplink and downlink such that, for example, uplink occurs for two immediately subsequent time slots TS (TS[j]& TS[j+1]), and that downlink occurs for two further subsequent time slots TS (TS[j+2] & TS[j+3]).
However, in TDD operation or TDD/TDMA operation of a cellular telecommunication network there exists a problem that depending on the specific case interference between uplink and downlink transmission may occur.
Now, some of these different problem cases are subsequently discussed.                Interference problems in asynchronous networks        
As set our above, a cellular network is provided by a plurality of base station transceiver devices BS, one per each cell. Such a network is operating asynchronously, if the respective base stations are not synchronized with respect to each other. If, however, the respective base stations are not synchronized, uplink and downlink transmission in neighboring or adjacent cells will occur at different times. Consequently, if for example a mobile transceiver device MS is close to a cell border, i.e. in an area where the transmission ranges of at least two base transceiver devices BS of adjacent cells are overlapping, uplink transmission (sending) causes interference to downlink reception close to that mobile transceiver device MS.                Asymmetric division between uplink and downlink        
In case the time division between uplink and downlink differs from cell to cell, i.e. between respective base stations BS1 and BS2 associated to a first and second cell, respectively, interference between uplink and downlink occurs at the cell borders of adjacent cells having asymmetry or different time division, respectively. FIG. 2 illustrates an example for such a situation of base stations BS1 and BS2. As shown, interference problems may occur in time slots TS[j], with j ∈ {1, 2, 5, 6}, since on one channel, uplink and downlink transmission is simultaneously present within the same time slot.                Several operators/several multiple access methods in the same band and in the same area        
The above description has heretofore been made for the case of a single cellular network only. That is, the cellular structure relates to the network operated by a single network operator only.
In practice, however, due to the increasing demand for mobile communication, it is rather the case that several network operators each run a cellular network in the same area. Moreover, these different networks may even use the same frequency band.
Therefore, even if the cells, i.e. the base stations in respective cells, within the cellular network of one operator are synchronized, there still remains the difficulty to synchronize the networks of several operators. Consequently, it is desirable that all networks within the same area and/or in the same frequency band should have the same asymmetry and adopt the same time division between uplink parts and downlink parts of a respective TDD/TDMA frame.
However, such a desired synchronization becomes even impossible, if the different operators are using different radio access techniques with different frame lengths. For example, in WB-TDMA (Wide Band Time Divisional Multiple Access) the frame length (of eight time slots) is 4.615 ms, while in WB-CBMA (Wide Band Code Divisional Multiple Access) the frame length is 10 ms. FIG. 3 illustrates such a situation for two base stations BS1, BS2 operating according to different radio access techniques. It has to be noted that the WB-CDMA frame is shown for time slots TS[0] to TS[5] only and is therefore illustrated incomplete. Nevertheless, it becomes clear that nearly two time slots of the WB-TDMA frame are transmitted during the period of a single time slot of the WB-CDMA frame. Hence, interference problems are very likely to occur in such a situation.
The above example has been described to illustrate the principle problems in such a case. However, it does not represent the current trend of actually implemented systems. According to the latest decision taken by ETSI (European Telecommunication Standards Institute), the principle of WB-TDMA is no longer considered for the transmission via the air interface (Um interface). Instead, for Europe it has been decided that frequency division duplex FDD is based on WB-CDMA and TDD is based on TD-CDMA (=time division CDMA=CDMA/TDMA). The frames length of both modes, FDD and TDD, is 10 ms. However, TDD mode is designed to form one frame of 16 time slots. Therefore, also in such a case of different numbers of time slots of respective different transmission modes during the same frame period, similar problems as those described herein above will apparently manifest.
Although networks operating according to DECT standard (Digital European Cordless Telephone) have recently been proposed, which are synchronized, the above mentioned second and third type of problems are still not solved in such networks.
Document WO 9322850 discloses a method for data transmission in a cellular telecommunication system as defined in the preambles of the independent claims 19 and 20. In detail, this document discloses a method of increasing interference diversity in an FDMA/TDMA-based cellular system, in which in sequential TDMA frames different time slots are used. A TDMA frame represents either a downlink frame or an uplink frame.
Furthermore, document WO 98 12678 A discloses a method of facilitating transmission level measurement. In this method, a base station moves the BCCH from one time slot to another so that the BCCH is sent in different time slots of successive frames.